/Engine/Source/Runtime/Engine/Classes/Engine/GameViewportClient.h
Adds a widget to the Slate viewport's game overlay (i.e for top-level debug tools) at the specified Z-order.
virtual void AddGameLayerWidget ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > ViewportContent, const int32 ZOrder )